In communication systems, data (or signal) is transferred by a transmitter over a physical media such as, for example, coaxial cable, twisted pair, or optical fiber. These physical media are non-ideal communication channels, which degrades the signal by causing attenuation and delay. The physical media also degrades the signal by adding noise and distortion to the signal. The degradation of the signal reduces the signal to noise ratio (SNR) of the signal. The reduced SNR decreases the dynamic range of a receiver, which receives the signal from the physical media.
Referring now to FIG. 1, a receiver 100 includes a variable gain amplifier (VGA) 104, an equalizer 108, a peak detector (PD) 116, a slicer 140, an analog offset controller (AOC) 112, a clock and data recovery circuit (CDR) 120, an automatic equalizer control (AEC) 124, an analog gain controller (AGC) 128, and a digital decoder 132. In the following description, the physical media will simply be referred to as a cable although the invention is applicable to other physical media as well.
The signal is transmitted over a cable 102, and the signal is then received by the receiver 100. The VGA 104 amplifies the signal to compensate for the frequency-independent loss, also known as resistive loss or flat loss.
The output of the VGA 104 is received by the equalizer 108. The equalizer 108 compensates for the frequency-dependent loss on the cable also known as cable loss. The equalizer 108 boosts the high frequency components of the signal to compensate for the cable loss.
The output of the equalizer 108 is received by the PD 116. In general, the PD 116, which receives an analog output from the equalizer 108, determines the peak of the equalized signal. The output of the equalizer 108 is also received by the AOC 112, which controls through the VGA 104 the differential offset of the receiver. Thus, the AOC 112 forms a feedback loop to adjust through the VGA 104 the differential offset of the receiver, driving the differential offset to 0 V level. The differential offset of the receiver is driven to a 0 V level in order to eliminate harmonic distortion inside the receiver 100.
As discussed before, the output of the equalizer 108 is received by the PD 116. The peak detector determines the peak of the equalized signal (i.e., the output of the equalizer 108) and sends the peak value to the slicer 140. The slicer 140 also receives the output of the equalizer 108. The slicer 140 functions as an analog to digital converter (e.g., a 2 bit A/D converter), which outputs a digital signal using the peak value.
The digital output of the slicer 140 is received by the CDR 120. The CDR 120 extracts the correct clock signal and data from the digital signal and also synchronizes the data and the clock signal. The output of the CDR 120 is received by the decoder 132, which decodes the signal according to a standard decoding scheme.
The analog output of the PD 116 is received by the AGC 128, which controls the gain of the VGA 104. The digital output of the slicer 140 and the output of the CDR 120 are received by the AEC 124, which controls the gain of the equalizer 108 by adjusting the equalizer coefficients or steps.
A signal, such as, for example, a digital signal (i.e., data) can be decomposed into a set of sinusoidal waves according to the Fourier theorem, each sinusoidal wave having a different frequency and amplitude. As discussed before, when a signal such as a sinusoidal wave travels over a cable, which is a non-ideal communication channel, it suffers from attenuation. The attenuation refers to the loss or decrease in amplitude of the signal. The sinusoidal wave is also subjected to a delay when traveling through the cable. The delay varies depending on the frequencies. Thus, the sinusoidal waves that form a digital signal will each be delayed by a varying degree. In general, the sinusoidal wave's attenuation and delay is directly proportional to the transmission distance. Thus, the longer the sinusoidal wave travels, the more attenuation and delay it suffers.
FIG. 2 illustrates the attenuation and delay of a square wave that travels through a 100 feet cable and a 1000 feet cable, respectively. As shown in FIG. 2, the square wave's attenuation and delay increase as the wave travels longer distance.
A signal traveling through a cable suffers two types of loss: (a) frequency dependent loss (i.e., cable loss); and (b) frequency independent (i.e., resistive loss or flat loss). A flat loss causes all frequencies of the signal to be attenuated by the same level. A cable loss causes different levels of attenuation to different frequencies. In general, a cable causes higher level of attenuation to high frequency signals and lower levels of attenuation to low frequency signals. Thus, when a square wave travels through a cable, its higher frequencies are attenuated more than lower frequencies. The cable also adds noise and distortion to the signal, which causes degradation of the signal.
In general, the receiver 100 (also known as an analog front-end receiver) is relied upon to restore transmitted signals after they travel through the cable. As shown in FIG. 1, the receiver 100 provides analog gain compensation, equalization and filtering. The equalizer 108 compensates for the frequency-dependent loss on the cable. The equalizer 108 is a combination of several blocks, each block having a low pass filter and an adjustable zero. The low pass filter is implemented with a fixed pole. In a typical equalizer, an all 0 code corresponds to a scenario when the pole and zero of each block are aligned and their individual effects are cancelled. When the equalizer coefficient is increased, the zero location moves towards lower frequency, resulting in a high pass function. The purpose of the equalizer is to create a high-pass function that is inverse of the low pass function of the cable, so that the combined frequency response of the cable and the equalizer is 0 dB (i.e., flat).
Ideal Equalization
FIGS. 3(a) and 3(b) illustrate an ideal equalization scheme, which restores a signal to its original state. In FIG. 3(a), the signal originates from a transmitter 304, travels through a cable 308, and is received by a receiver 312. The signal is subjected to cable loss as it travels through the cable 308. FIG. 3(a) shows that the original waveform is attenuated due to cable loss and is then restored or equalized by an equalizer inside the receiver 312.
FIG. 3(b) illustrates the cable loss characteristics, the equalization frequency response, and the overall transfer function. The cable loss characteristics indicate attenuation of the high frequency components of the signal due to the low pass characteristics. The equalizer is adjusted so that the high frequency components of the signal are amplified, thereby canceling the cable loss effect and restoring the signal to its original state. The overall transfer function after the equalization is the 0 dB line.
In reality, however, the equalizer doesn't completely cancel the effect of the cable loss on a signal. The equalizer either under equalizes or over equalizes.
Under Equalization
If the equalizer's high frequency boost is inadequate to cancel the loss caused by the cable, the resulting signal will be under equalized. An under equalized signal is not fully restored to its original form. FIGS. 4(a) and 4(b) illustrate an under equalization scenario. In FIG. 4(a), a signal originating from a transmitter 404 travels over a cable 408 and is subjected to cable loss. The cable loss causes attenuation of the high frequency contents of the signal. An equalizer inside the receiver 412 attempts to restore the signal to its original shape. However, as shown in FIG. 4(a), the equalizer's high frequency boost is inadequate to restore the attenuation due to the cable loss. Thus, the signal after equalization is under equalized. FIG. 4(b) shows the equalization frequency response, cable loss characteristics and overall transfer function. Due to inadequate equalization, the transfer function indicates that the high frequency components are not completely restored.
Over-Equalization
If the equalizer provides more high frequency boost than necessary to cancel the cable loss, the resulting signal is over equalized. An over equalized signal contains ringing. FIGS. 5(a) and 5(b) illustrate an over equalization scenario.
In FIG. 5(a), a signal originating from a transmitter 504 travels over a cable 508 and is subjected to cable loss. The cable loss causes attenuation of the high frequency contents of the signal. An equalizer inside a receiver 512 attempts to restore the signal to its original shape. However, as shown in FIG. 5(a), the equalizer provides excess high frequency boost, which causes a ringing waveform.
FIG. 5(b) shows the equalization frequency response, cable loss characteristics and overall transfer function. Due to excessive equalization, the transfer function indicates that the resulting signal includes excessive high frequency boost, thus resulting in a ringing waveform.
Both under equalized and over equalized signals are undesirable in communication systems. An over equalized signal causes error in a peak detector, resulting in bit errors. An under equalized signal causes inter-symbol-interference (ISI). The problems caused by under equalization and over equalization will be discussed further below.
FIGS. 6(a)-6(c) illustrate the problems due to over equalization. In FIG. 6(a), the original signal consists of 0, 1, 0, −1. FIG. 6(b) illustrates the resulting signal after over equalization. Due to over equalization, the signal contains significant ringing. FIG. 6(c) illustrates the signal after being processed by a peak detector. The output of the peak detector is an incorrectly restored signal consisting of 1, 0, 1, −1, 0, 1, −1, 0, −1, 1.
FIGS. 7(a)-7(c) illustrate the problem due to under equalization. In FIG. 7(a), the original signal consists of 0, 1, 0, −1. FIG. 7(b) illustrates the resulting signal after under equalization. FIG. 7(c) illustrates the waveform after being processed by a peak detector. The output of the peak detector consists of 1, 0, −1, 0. Thus, the output of the peak detector contains significant error.
Dynamic Range of a Receiver
The dynamic range of the receiver 100 is related to the receiver reach. The receiver reach is defined as the maximum cable length over which the signal can travel and yet remains within acceptable bit error degradation. A large dynamic range of the receiver provides a large receiver reach. As discussed before, when a signal travels a large distance, it suffers from attenuation and delay. The attenuation and delay causes bit error at the output of the slicer 140, thereby having a negative effect on the receiver reach.
As shown in FIG. 1, the AGC 128 receives the output of the PD 116. The signal processed by the PD 116 has already been compensated by the VGA 104 for flat loss and by the equalizer 108 for cable loss. The equalizer 108 compensates for the frequency dependent loss in the cable due to the non-ideal characteristics of the cable. The equalizer 108 boosts the high frequency components of the signal. However, the high frequency boost applied by the equalizer 108 also increases the amplitude of the signal. The AEC 124 controls the equalizer steps or coefficient values. When there is a high cable loss, the equalizer coefficient values are set to a high value to provide large high frequency boost. The large high frequency boost increases the amplitude of the signal, which is processed by the PD 116 and subsequently received by the AGC 128. Since the signal received by the AGC 128 has high amplitude, the AGC 128 decreases the gain of the VGA 104.
If the VGA's gain is decreased significantly, the signal to noise ratio (SNR) at the input of the equalizer 108 is reduced, thus degrading the signal quality. A degradation of the SNR limits the receiver's ability to recover signals over a long cable length, thereby reducing the dynamic range of the receiver. Also, the addition of the flat loss to the cable loss causes the AGC to inadequately compensate for the flat loss due to the large high frequency boost of the equalizer, thus degrading the SNR further.
Accordingly, there is a need for a receiver with an enhanced dynamic range. There is a need for an equalization method and system that compensate for the cable loss without degrading the SNR of a signal.